When I've played live I tend to be nervous waiting for the tournament to start. Obviously the anticipation of the event can get your blood pumping - which isn't always a good thing.
Online or live cash games I'm not nervous. Even when I've played in tournaments with big buy-ins ($215, $430) I wasn't nervous online - just focused on the task at hand and formulating what my initial plan of attack would be.
In a pot, I don't get nervous when I see AA anymore - now I'm thinking about what my move will be to play the hand given my position, the size of the pot, the table image I have and the table image overall.
Nervousness creates focus but too much builds excitement - the last thing you want to do entering a pot. Play with thought and process, not because you see two shiny kings.